The invention relates to a filter device, which is intended to separate a liquid phase from a solid phase in a biological fluid, and to its application.
This device can be used, for example, in the separation of a blood plasma cryoprecipitate. It is known that when frozen blood plasma is thawed to a temperature generally of between +1 and +4.degree. C., not only is a liquid phase obtained, but also a residue which remains solid at this temperature and which is called cryoprecipitate. The cryoprecipitate includes substances which are of therapeutic interest, such as clotting factor VIII, or the proteins which can be coagulated by thrombin, and in particular fibrinogen, fibronectin and factor XIII. Factor VIII is used in the treatment of haemophilia type A. The proteins which can be coagulated by thrombin are used, in the form of the cryoprecipitate, in particular as a "biological adhesive"; see, for example, the patent FR 2 448 900. Biological adhesives are used in surgery on account of their haemostatic and adhesive properties. Their use makes it possible to avoid forming sutures or using synthetic adhesives which are potentially toxic.
The cryoprecipitate is usually obtained from one or more samples of whole blood. Blood is taken from a healthy donor, for example, or from a patient who is registered in an autotransfusion programme, with the aid of a blood collection system comprising a number of transfer bags which are connected to each other via tubing. The blood collection bag contains an anticoagulant. The plasma is separated by centrifuging and is discharged into one of the other bags. The plasma is then frozen, and subsequently thawed to a temperature which permits fusion of the plasma, but which is sufficiently low to prevent the cryoprecipitate from solubilizing. For the preparation of a cryoprecipitate, reference may be made, for example, to the article by Masure in Vox Sang. 16, 1-9 (1969).
Special techniques can be employed to prevent contamination of the product during the preparation. The preparation is preferably done in a closed circuit, the transfers of liquids being carried out in a series of bags and tubings which have previously been connected to one another and sterilized before use.
The separation of the liquid phase and the solid phase, that is to say the separation of the residual plasma and the cryoprecipitate, can be carried out either by centrifuging or by filtration.
Centrifuging has a number of drawbacks which are inherent to this method, that is to say, in addition to the requirement to have a centrifuge available, there is the difficulty of decanting the supernatant without disturbing the centrifugation concentrate, and the difficulty of recovering the centrifugation concentrate after decantation, since this concentrate occupies a small volume, while being contained in a vessel of large volume.
Filtration also involves technical difficulties. It is necessary, in the first instance, to prevent clogging of the filter. Porous filtering membranes can be used as filters, but in practice these necessitate the use of an auxiliary pump. French Patent 2 283 700 has proposed using a thickness filter, consisting, for example, of fibrous materials forming an irregular, three-dimensional entanglement. However, with thickness filters, the difficulty lies in recovering the cryoprecipitate; in practice, it is necessary in this case to redissolve the cryoprecipitate in a suitable solvent.